Medical ventilator triggering and cycling method and mechanism

ABSTRACT

A medical ventilator system and method that triggers, cycles, or both based on patient effort, which is determined from cross-correlating patient flow and patient pressure. The medical ventilator is also controlled such that sensitivity to a patient initiated trigger increases as the expiratory phase of the breathing cycle progresses. The present invention also provides adaptive adjustment of cycling criteria to optimize the cycling operation.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) fromprovisional U.S. patent application No. 60/238,387 filed Oct. 6, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a medical ventilator with improvedspontaneous triggering and cycling and to an improved method oftriggering and cycling such a ventilator. In particular, the presentinvention pertains to a ventilator with one or more of the followingfeatures: (1) sensitivity to a patient initiated trigger that increasesas the expiratory phase of the breathing cycle progresses, (2) adaptiveadjustment of cycling criteria to optimize the cycling operation, and(3) triggering, cycling, or both based on patient effort, which isdetermined from cross-correlating multiple patient parameters.

2. Description of the Related Art

It is known to utilize a conventional ventilator or pressure supportdevice to deliver a fluid, such as oxygen, air or other oxygen orbreathing gas mixture, to an airway of patient to augment or substitutethe patient's own ventilatory effort. It is further known to operate aconventional ventilator in a variety of modes to control the four basicoperations of a ventilator, which are: 1) the trigger point, which isthe transition from the expiratory to the inspiratory phase of theventilatory cycle; 2) the inspiratory phase where the ventilatordelivers the flow of breathing gas; 3) the cycle point, which is thetransition from the inspiratory phase to the expiratory phase, and 4)the expiratory phase. There are four primary variables or parametersthat are typically monitored and used to control how a ventilatorperforms one or more of these four operations. These variables are thevolume, pressure, flow of fluid to or from the patient, and time.

In a typical life support situation, where there is substantially nospontaneous respiratory effort by the patient, a controlled mode ofventilation is provided, where the ventilator assumes fullresponsibility for ventilating the patient. In this mode of ventilation,the trigger and cycle point of the ventilator are determined based ontime. In other situations, where the patient exhibits some degree ofspontaneous respiratory effort, an assist mode or a support mode ofventilation is typically provided. Both of these modes of ventilationcause the ventilator to augment or assist in the patient's ownrespiratory efforts. In the assist mode, the determination of theventilator trigger point is based on the action of the patient and thedetermination of the cycle point is determined based on time. In thesupport mode, both the trigger and the cycle points are patient basedand not based on time. It is also known to use a combination of thesetwo modes, referred to as an assist/control mode of ventilation. In thismode of ventilation, the ventilator triggers an inspiratory flow only ifthe patient fails to initiate a respiratory effort for a period of time.Thus, the trigger point is based on either a patient action or on time,if there is no patient action within a certain period of time.

In the assist, support, and assist/control modes of ventilation, it isimportant that the operation of the ventilator is synchronized with thepatient's spontaneous respiratory effort, so that the ventilatortriggers the inspiratory flow of breathing gas at or near the time thepatient begins his or her inspiratory effort, and cycles to theexpiratory phase of the breathing pattern at an appropriate time,preferably when the patient begins his or her expiratory phase of thebreathing cycle. Conventional ventilators operating in an assist,support, or assist/control mode of ventilation typically monitor onlyone patient parameter, such as the pressure, flow, or volume, and usethis single monitored parameter as a variable in determining when tospontaneously trigger the delivery of the inspiratory flow. Typically,the monitored parameter is compared to a threshold, and if the thresholdis exceeded, the transition from expiration to inspiration (trigger) orfrom inspiration to expiration (cycle) is initiated. In other pressuresupport devices, the current value of the monitored parameter iscompared to a previous value of the same parameter, so that theventilator triggers or cycles based on the result of this comparison.U.S. Pat. No. 5,632,269 to Zdrojkowski et al. teaches this techniquereferred to as “shape triggering.”

This one-dimensional, i.e., one parameter, comparison of eitherpressure, flow, or volume to a trigger threshold is disadvantageous inthat it is susceptible to random fluctuations in the monitoredparameter, which may result in false triggers or cycles. In which case,an operator must intervene to reduce the trigger and/or cycle thresholdsor ventilator sensitivity. However, reducing the ventilator'ssensitivity can result in a greater amount of patient effort beingneeded before a spontaneous patient inspiration or expiration isdetected, which is also disadvantageous, because a patient on aventilator often has a weakened respiratory system to begin with.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amedical ventilator system that overcomes the shortcomings ofconventional ventilators with improved triggering and/or cyclingcapability. This object is achieved according to one embodiment of thepresent invention by providing a ventilator system that includes a gasflow generator adapted to provide a flow of breathing gas, a gas flowcontroller that controls the flow of breathing gas delivered to thepatient responsive to a control signal, a patient circuit adapted tocommunicate the flow of breathing with an airway of the patient, a flowsensor adapted to measure the flow of breathing gas in the patientcircuit and to output a first flow signal indicative thereof, a pressuresensor adapted to measure a pressure of the flow of breathing gas in thepatient circuit and to output a first pressure signal indicativethereof, and an exhaust assembly adapted to communicate gas from withinthe patient circuit to ambient atmosphere. The ventilator system alsoincludes a controller that receives the first flow signal and the firstpressure signal and outputs the control signal that controls the flow ofbreathing gas delivered to the patient circuit by the pressuregenerating system and, hence, the flow of breathing gas at a patient'sairway. In one embodiment, the controller detects the onset of theinspiratory phase of a patient's breathing cycle for triggering theinspiratory flow of breathing gas based on such a patient's inspiratoryeffort, which is determined based on both the first flow signal and thefirst pressure signal.

According to a further embodiment of the present invention, thecontroller arms or makes available for activation a plurality oftriggering mechanisms over an expiratory phase of a breathing cycle toincrease the ventilator system sensitivity to a patient initiatedtrigger as the expiratory phase of the breathing cycle progresses.

In a still further embodiment, the controller detects the onset of theexpiratory phase for cycling the ventilator based on such a patient'sexpiratory effort, which is determined based on both the first flowsignal and the first pressure signal. This cycling feature of thepresent invention can be done alone or in combination with thetriggering feature noted above.

In yet another embodiment of the present invention, the controllerdynamically adjusts the cycling threshold criteria on a breath by breathbasis so that the ventilator cycles more closely in synchronization withthe patient's expiratory effort. In this embodiment, the ventilatorsystem monitors the patient pressure P_(patient) and, more particularly,its rate of change at the end of the inspiratory phase, as well aschanges in the patient flow Q_(patient) at the beginning portion of theexpiratory phase to determine if the ventilator cycling for that breathoccurred before or after the patient began exhalation, and dynamicallyadjusts the cycling threshold criteria in the next breath to account forthe cycling synchronization error in the previous breath.

It is yet another object of the present invention to provide a method oftriggering or cycling a medical ventilator that does not suffer from thedisadvantages associated with conventional triggering and cyclingtechniques. This object is achieved by providing a method that includes:(1) generating a flow of breathing gas, (2) providing the flow ofbreathing gas to a patient via a patient circuit, (3) controlling theflow of breathing gas delivered to a patient responsive to a controlsignal, (4) measuring the flow of breathing in the patient circuit andoutputting a first flow signal indicative thereof, (5) measuring apressure of the flow of breathing gas in the patient circuit andoutputting a first pressure signal indicative thereof, (6) communicatinggas from within the patient circuit to ambient atmosphere, (7) detectingthe onset of the inspiratory phase of a patient's breathing cycle fortriggering an inspiratory flow of breathing gas based on the patient'sinspiratory effort, which is determined based on both the first flowsignal and the first pressure signal, and (8) detecting the onset of theexpiratory phase of a patient's breathing cycle for cycling purposebased on the patient's expiratory effort, which is also determined basedon both the first flow signal and the first pressure signal. It shouldbe noted that triggering and cycling can be done independently or theycan both be done during the appropriate stages of the breathing cycle.

According to a further embodiment of the present invention, a pluralityof triggering mechanisms are made active during different stages of theexpiratory phase of the patient's breathing cycle to increase theventilator system sensitivity to a patient initiated trigger as theexpiratory phase of the breathing cycle progresses.

In yet another embodiment, the present invention provides a medicalventilator system that cycles from providing an inspiratory flow ofbreathing gas to allowing an expiratory flow by comparing the patientflow and a cycle threshold criteria. The system further dynamicallyadjusts the cycling threshold criteria on a breath by breath basis basedon changes in patient pressure P_(patient) at the end portion of theinspiratory phase and based on changes in the patient flow Q_(patient)at the beginning portion of the expiratory phase, which are indicativeof whether the ventilator cycling for that breath occurred before orafter the patient began exhalation. In this embodiment, the cyclingthreshold criteria are dynamically adjusted in the next breath toaccount for the cycling synchronization error in the previous breath.

These and other objects, features and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a ventilator system adapted toimplement the triggering and cycling techniques of the presentinvention;

FIG. 2 is a waveform illustrating a typical patient flow of two normal,spontaneous respiratory cycles;

FIG. 3 is a waveform illustrating patient flow and patient pressureduring a typical triggering process implemented by the ventilator systemof FIG. 1 according to the principles of the present invention; and

FIG. 4 is a flowchart of a triggering process implemented by theventilator system of FIG. 1.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THEINVENTION

FIG. 1 schematically illustrates an exemplary embodiment of a ventilatorsystem 30 according to the principles of the present invention.Ventilator system 30 is adapted to operate in an invasive mode, wherethe patient is typically intubated, or in a non-invasive mode, where thepatient is not intubated. The basic components in ventilator 30correspond to those found in a conventional ventilator, such as theEsprit® Ventilator manufactured by Respironics, Inc. of Pittsburgh, Pa.,with the novel aspect of the present invention being the techniques usedby the ventilator to trigger and/or cycle, such as utilizing recognitionand quantification of physiologic-based concomitant multi-signalpatterns, as opposed to a single signal pattern of conventionalventilators, for triggering purposes, cycling purposes, or both.

A. Ventilator System Hardware

Ventilator system 30 includes a primary gas flow delivery system,generally indicated at 32, which includes a pressure generator 34, apressure/flow control element 36, and a flow sensor 38. Pressuregenerator 34 receives a flow of breathing gas, such as air, oxygen, oran oxygen mixture, as indicated by arrow A, through an optional muffler40 or other noise suppression device from a supply of breathing gas (notshown). Pressure generator 34 elevates the pressure of the receivedbreathing gas to generate a flow of breathing gas, as indicated byarrows B for delivery to an airway 42 of a patient 44.

In a preferred embodiment of the present invention, the pressuregenerator is a blower, which uses an impeller rotated by a motor togenerate the flow of breathing gas at an elevated pressure relative tothe ambient atmospheric pressure. It is to be understood, however, thepresent invention contemplates other devices and techniques forelevating or generating the flow of breathing gas, such as a piston, abellows, and helical or drag compressor. The present invention furthercontemplates that the pressure generator can be a source of pressurizedgas, such as air, oxygen or a gas mixture from a pressurized tank, acompressor, or from the wall outlet typically provided in a modernhospital. It can be appreciated that if the source of pressurized gas isfrom these latter sources, muffler 40 and pressure generator 34 can beeliminated and a pressure regulator may be required.

Pressure/flow control element 36, which is preferably downstream ofpressure generator 34, controls the pressure or flow of breathing gasdelivered to the patient. In a preferred embodiment of the presentinvention, pressure/flow control element 36 is a valve operating underthe control of a processor 46. It is to be understood, however, that thepresent invention contemplates other techniques for controlling the flowof breathing gas delivered to an airway 42 of patient 44 by primary gasflow delivery system 32, such as modulating the flow of gas delivered topressure generator, modulating the operating speed of pressuregenerator, or any combination of these techniques.

Flow sensor 38 is any suitable flow sensing device capable ofquantitatively measuring the amount of fluid flowing therethrough andoutputting a flow signal Q_(primary) indicative thereof. In theillustrated exemplary embodiment, flow sensor 38 is downstream ofpressure/flow controller 36. It is to be understood, however, that otherlocations and techniques for measuring the flow of breathing gasdelivered to the patient by primary gas flow delivery system 32 arecontemplated by the present invention, such as based on the operation ofpressure/flow controller 36 or the energy provided to pressure generator34.

In the illustrated exemplary embodiment, the flow of breathing gas,after being measured by flow sensor 38, is provided to an inhalationmanifold 48 and delivered to patient 44 via a patient circuit 50. In apreferred embodiment of the present invention, patient circuit 50 is atwo-limb circuit having an inspiratory limb 52 for carrying gas to thepatient, as indicated by arrow C, and an expiratory limb 54 for carryinggas from the patient, as indicated by arrow D, to an exhaust assembly,generally indicated at 56. A patient interface device 58 communicatesthe patient circuit with the airway of the patient. The presentinvention contemplates that patient interface device 58 is any device,either invasive or non-invasive, suitable for communicating a flow ofbreathing gas from the patient circuit to an airway of the patient.Examples of suitable patient interface devices include a nasal mask,nasal/oral mask (which is shown in FIG. 1), full-face mask, trachealtube, endotracheal tube, and nasal pillow.

Exhaust assembly 56 monitors and/or controls the venting of exhaustfluids to atmosphere, as indicated by arrow E, from expiratory limb 54and includes a pressure sensor 60, an exhaust flow sensor 62 and anexhaust flow control element 64. Pressure sensor 60 measures thepressure P_(prox) in expiratory limb 60 at a location proximal to theexhaust vent. For present purposes, pressure P_(prox) is considered tocorrespond to the pressure at the patient P_(patient). Pressure signalP_(prox) is provided to processor 46. Of course, the pressure at thepatient P_(patient) can be measured directly via a pressure port in thepatient interface device, for example.

Exhaust flow sensor 62, like flow sensor 38, is any suitable flowmeasuring device capable of quantitatively measuring the amount of fluidflowing therethrough and outputting a flow signal Q_(exhaust) indicativethereof.

Exhaust flow control element 64 is preferably an active exhaust valvethat can be selectively actuated to regulate the venting of exhaust gasto atmosphere under the control of processor 46. In particular, exhaustflow control element 64 preferably prevents fluid from exhausting toatmosphere when pressurized fluid is supplied to patient 44, i.e.,during the inspiratory phase, and allows gas to escape to atmosphere ata controlled rate when the supply of pressurized fluid to patient 44 isterminated or reduced, i.e., during the expiratory phase. The activeexhaust assembly preferably controls the flow of exhaust gas toatmosphere to control the positive end exhalation pressure (“PEEP”) inthe patient.

Ventilator system 30 in FIG. 1 includes an optional oxygen or secondarygas delivery system, generally indicated at 66, for delivering asupplemental or secondary gas flow, indicated by arrow F, concomitantlywith the primary gas flow B. The oxygen or other secondary gas flowindicated by arrow G is delivered from a gas source (not shown), such asan oxygen tank or a wall outlet in a hospital, to a secondary gas flowcontrol element 68, which is typically a valve. The pressure and/or flowof the secondary gas flow F is regulated by secondary gas flow controlelement 68 under the control of processor 46. The second gas flow isprovided to inhalation manifold 48 where it is introduced or mixed withthe primary gas flow, which together form the flow of breathing gas(arrow C) delivered to the patient. A secondary flow sensor 70 isprovided for measuring the flow of secondary gas Q_(secondary) providedto the inhalation manifold.

It should be noted that secondary gas delivery system 66 is optional andcan be eliminated. However, in many practical ventilatorimplementations, it is desirable to deliver a flow of gas to a patienthaving a higher oxygen content than that available from the ambientatmosphere. In addition, medical gasses other than oxygen, can bedelivered by secondary gas delivery system 66.

It is to be understood that the schematic diagram of a ventilator systemshown in FIG. 1 is not intended to be a complete and exhaustivedescription of ventilator system, but is intended to describe the keycomponents of the ventilator, especially those necessary to carry outthe unique triggering and cycling techniques of the present invention.Those skilled in the art would understand, for example, that a medicalventilator system could also include features such as an input/outputdevice for setting the operating parameters of the system, alarms(audible or visual) for signaling conditions of the patient orventilator to an operator, as well as ancillary elements connected tothe patient circuit, such as a humidifier, bacteria filter, anaspiration catheter, and a tracheal gas insufflation catheter, to name afew.

B. Ventilator System Operation

The operation of ventilator system 30 is discussed below with referenceto FIGS. 3-4 and with continuing reference back to FIG. 1. However,before discussing the operation in detail, it is helpful to establishsome basic terminology regarding a ventilated patient. To this end, FIG.2 shows an exemplary waveform 72, illustrating the patient flowQ_(patient) for two respiratory or breathing cycles in a normal,spontaneously breathing patient. Each breathing cycle T_(breath) can bedivided into four parts: (1) the transition from exhalation toinhalation, i.e., trigger point 74, (2) the inhalation or inspiratoryphase, T_(insp), (3) the transition from inhalation to exhalation, i.e.,cycle point 76, and (4) the exhalation or expiratory phase T_(exp).

The triggering and cycling techniques of the present invention controlthe process by which the ventilator system triggers from expiration toinspiration and cycles from inspiration to expiration, respectively, sothat these events are synchronized with the breathing cycle of aspontaneously breathing patient. It is to be understood that thetriggering and cycling techniques can be used independently of oneanother and can be used in conjunction with other functions present in aconventional ventilator, such as a timed backup breath should thepatient fail to trigger the inspiratory flow after a set period of time,alarms, and other ventilation or pressure support modes. For example, ina pressure support or pressure assist mode of ventilation, one or bothof the triggering and cycling techniques described herein are used. In avolume controlled mode of ventilation, only the triggering techniques ofthe present invention are utilized because cycling is timed, notspontaneous.

The triggering and cycling techniques of the present invention can alsobe used separately or concurrently in other modes of ventilation orpressure support such as: (1) proportional assist ventilation (PAV)mode, as taught, for example, in U.S. Pat. Nos. 5,044,362 and 5,107,830,both to Younes, the contents of each of which are incorporated herein byreference; (2) proportional positive airway pressure (PPAP) support astaught, for example, in U.S. Pat. Nos. 5,535,738; 5,794,615; and6,105,575 to Estes et al., the contents of each of which areincorporated herein by reference; and (3) bi-level pressure support astaught, for example, by U.S. Pat. No. 5,148,802 to Sanders et al., U.S.Pat. No. 5,313,937 to Zdrojkowski et al., U.S. Pat. No. 5,433,193 toSanders et al., U.S. Pat. No. 5,632,269 to Zdrojkowski et al., U.S. Pat.No. 5,803,065 to Zdrojkowski et al., and U.S. Pat. No. 6,029,664 toZdrojkowski et al., the contents of each of which are incorporated byreference into the present invention.

As discussed in greater detail below, the ventilator triggering andcycling techniques of the present invention uses cross-correlatorypatterns of patient flow Q_(patient) and patient pressure P_(patient)deviations from steady state as metrics for determining spontaneous,i.e., patient initiated, breath phase transitions. The use of bothpressure and flow is believed to present a more accurate indication ofthe patient's respiratory effort as a trigger or cycle than is possiblewith conventional triggering or cycling techniques. In addition, thetriggering and cycling techniques of the present invention efficientlydetect patient initiated trigger and cycle events without erroneouslyinterpreting noise or other system aberrations as such events.

Referring now to FIG. 3, which shows patient flow Q_(patient) 78 andpatient pressure P_(patient) 80, the present inventors noted that in thepresence of a base flow, a patient's inspiratory effort, which begins atpoint 82, is indicated by a decrease in the patient pressure from thecontrolled pressure level 84, typically the PEEP level, followed byincreasing flow into the lung. This decrease in pressure is indicated at86 in FIG. 3 and the nearly concurrent increase in flow is indicated at88. It should be noted that there is a small delay between the inceptionof the patient pressure drop and the rise in patient flow. The length ofthe delay can vary depending on the patient. For example, in patientssuffering from COPD, the delay is longer than in other patients.Shortly, thereafter, at point 90, the ventilator triggers, and thepatient flow and pressure increase. In one embodiment, the presentinvention uses this observable pressure drop and flow increase totrigger the ventilator, because this pressure-flow pattern indicates thepatient has initiated an inspiratory effort.

The present invention contemplates using a similar, yet opposite,pattern that takes place at the end of the inspiratory phase of thebreathing cycle to cycle the ventilator system from the inspiratoryphase to the expiratory phase. More specifically, the present inventorsnoted that in a ventilated patient whose expiratory flow is controlledby an exhaust valve, the patient's expiratory effort, which begins atpoint 92, is indicated by a decrease in the patient flow, indicated at94, followed by an increase in patient pressure, indicated at 96. Thispressure-flow pattern indicative of a cycle event can be used to cyclethe ventilator at point 98, causing the patient flow and pressure todecrease as expiration commences.

In another embodiment of the present invention, cycling the ventilatoris accomplished based on patient flow and pressure changes at the end ofinspiration. In this embodiment, discussed in detail below, the cyclingthreshold is dynamically altered on a breath by breath basis to maximizepatient comfort as well as patient-machine synchrony.

The ventilator system of the present invention also provides othertriggering mechanisms that are made active, i.e., begin searching for atriggering event, at different stages during the progression of thepatient's expiratory cycle. These other triggering mechanisms preferablyrun concurrently with the pressure-flow triggering process of thepresent invention, so that a spontaneous trigger of the ventilator cantake place whenever one of the trigger events occurs. This use ofmultiple triggering mechanisms becoming active due to the conditionsthat occur at various stages of the expiratory phase effectively causesthe ventilator system's sensitivity to a patient initiated inspiratoryeffort for triggering purposes to be low at the start of the expiratoryphase of the breathing cycle and increase as the patient progressesthrough the expiratory phase. As a result, false triggers are minimizedwhile maximizing the system's responsiveness to the patient'sinspiratory effort.

1. Patient Flow, Patient Pressure, and Leak

For purposes of the present invention, the patient pressure P_(patient)is deemed to correspond to the proximal pressure P_(prox) measured bypressure sensor 60. That is, P_(patient)=P_(prox). Of course,P_(patient) could be measured directly at the patient using anyconventional technique. In ventilator system 30 shown in FIG. 1, patientflow Q_(patient) is not measured directly in the same fashion as patientpressure, because it is not practical to place a flow meter at thepatient's airway openings. Therefore, for this embodiment of the presentinvention, the actual patient flow Q_(patient) is determined from theflows measured by flow sensors 38, 62, and 70. It is to be understood,however, that ventilator system 30 can be modified to provide a flowsensor near the patient. In which case, the patient flow Q_(patient) ismeasured directly.

In a closed system, i.e., a system with substantially no or at leastminimal, negligible leaks, such as ventilator system 30 of FIG. 1, theinstantaneous patient flow Q_(patient) is defined as:

Q _(patient) =Q _(primary) +Q _(secondary) −Q _(exhaust).  (1)

In this case, patient flow is directly determined without taking intoconsideration any systems leaks. This paradigm is generally applicableto a typical invasive ventilation system, because, in such a system,when set up properly, leaks are specifically intended to be minimized.

The present invention, however, contemplates including losses due toleaks into equation (1) so that the actual patient flow is determinedmore accurately by taking into consideration intentional andunintentional leaks. Intentional leaks, can include leaks through anexhaust port specifically provided in the patient circuit and/or patientinterface to vent exhaust gas from the patient to atmosphere. Asingle-limb, non-invasive ventilator or pressure support device mayinclude this type of exhaust port to allow the patient's expired gas tovent to atmosphere. Unintentional leaks can occur, for example, at thepatient interface contact, such as between a mask seal and the patient'sskin, and at couplings in the patient circuit. Taking into considerationleaks, equation (1) becomes:

 P _(patient) =Q _(primary) +Q _(secondary) −Q _(exhaust) −Q_(leak),  (2)

where Q_(leak) is the instantaneous leak flow, including intentional andunintentional leaks.

The present invention contemplates using any conventional technique forcalculating leak flow Q_(leak), such as those taught by U.S. Pat. No.5,148,802 to Sanders et al., U.S. Pat. No. 5,313,937 to Zdrojkowski etal., U.S. Pat. No. 5,433,193 to Sanders et al., U.S. Pat. No. 5,632,269to Zdrojkowski et al., U.S. Pat. No. 5,803,065 to Zdrojkowski et al.,and U.S. Pat. No. 6,029,664 to Zdrojkowski et al., and pending U.S.patent application Ser. No. 09/586,054 to Frank et al., the contents ofeach of which are incorporated by reference into the present invention.Although one can refer to one or more of these references for adescription of techniques for detecting and estimating leak and managingthe delivery of breathing gas to the patient in the presence of leaks, abrief description of this process is provided below for the sake ofcompleteness.

According to one leak estimation technique, Q_(leak) in equation (2) atany given moments is determined as:

Q _(leak) =LF{square root over (P_(patient))},  (3)

where LF is a leak factor that is preferably calculated for each breathas: $\begin{matrix}{{LF} = {\frac{\int_{0}^{T_{breath}}{\left( {Q_{primary} + Q_{Secondary} - Q_{exhaust}} \right){t}}}{\int_{0}^{T_{breath}}{\sqrt{P_{patient}}{t}}}.}} & (4)\end{matrix}$

During a breathing cycle T_(breath), processor 46 monitors the flowsignals Q_(primary), Q_(secondary), and Q_(exhaust) from flow sensors38, 62, and 70 and the pressure signal P_(prox) from pressure sensor 60.Using this information gathered over a complete breathing cycle,processor 46 determines the value for∫₀^(T_(breath))(Q_(primary) + Q_(secondary) − Q_(exhaust))(t)  t  and  ${\int_{0}^{T_{breath}}{\sqrt{P_{patient}(t)}\quad {t}}},$

which are the terms in the numerator and denominator, respectively, forequation (4).

To determine a value for Q_(leak) at any given instant in a breathcycle, processor 46 solves equation (3) utilizing the known value for LFfrom equation (4), which was calculated from the previous breathingcycle, and the measured flows and pressure at that instant. In anexemplary embodiment of the present invention, processor 46 samples thesignals generated by flow sensors 38, 62 and 70 and pressure sensor 60 aplurality of times, for example, 100 samples per breath cycle or onceevery processing cycle, which takes place every 5 milliseconds (ms), tocompute the patient flow from equation (2) essentially continuously.

It is to be understood that the present invention also contemplatesusing an average value of LF, rather than the leak factor determined inthe immediately preceding breath cycle. For example, the leak factor foreach of the last n breath(s) can be calculated and the average leakfactor over the n breath(s) can be used in equation (3) to determineleak, where n is an integer. The present invention also contemplatesthat numerator, the denominator, or both in equation (4) can bedetermined from an average of these values determined during the last nbreaths. In addition, the changes in leak factor can be made gradually,so that sudden changes in leak flow, do not result in abrupt changes inthe leak factor or average leak rate used by the present invention.

While the technique described above for calculating the instantaneouspatient flow Q_(patient) is effective, it requires repeatedrecalculating of the leak factor LF, and determining the leak rateQ_(leak) for each breathing cycle in order to determine the patient flowaccurately. The present invention, however, contemplates anothertechnique for calculating the patient flow Q1 _(pateint) under certainconditions that does not require a leak rate calculation. Namely, ifventilation system 30 is in a constant leak condition, which will occurwhere the patient pressure remains constant, the patient flow Q1_(patient) is determined as follows. First, the net flow NetFlow(n) isdetermined as:

NetFlow(n)=Q _(total) −Q _(exhaust),  (5)

where Q_(total)=Q_(primary)+Q_(secondary), which is the total flowoutput under the control of the ventilator system.

Over a 100 ms moving window of time, a volume (Volume(n)) is calculatedas follows: $\begin{matrix}{{{{Volume}(n)} = {0.005*{\sum\limits_{n - 19}^{n}{{NetFlow}(k)}}}},} & (6)\end{matrix}$

where n is processing cycle of 5 ms. Consecutive volume differentials(Volume Differentials(n)) are then calculated over a moving 50 ms windowas:

Volume Differential(n)=Volume(n)−Volume(n−10).  (7)

According to this patient flow measurement technique, the net flow tothe patient (equation (6)), which may or may not include a leak, fromtwo different moving windows that are spaced closely together, arecompared to one another (equation (7)). In doing this, the leak, whichis constant and, thus, the same in each moving window, is cancelled out,so that the resulting difference, i.e., the Volume Differentialrepresents the volume of fluid delivered to or received from thepatient. The patient flow Q1 _(patient) is then determined on acontinuous basis as:

Q1 _(patient)=Volume Differential(n)/0.050+Q1 _(patient(prior)),  (8)

where Q1 _(patient(prior)) is the patient flow determined in theprevious processing window.

The number, 0.050 in the denominator is selected because the window forthe Volume calculation is a 50 ms window, with Q1 _(patient) beingexpressed in liters per second. As noted above, this technique forcalculating patient flow is advantageous in that the patient flow can becalculated while effectively ignoring leak flow. However, the pressuremust be stable in order to use this patient flow calculating technique.

Those skilled in the art can appreciate that either technique fordetermining patient flow, e.g., by determining patient flow Q_(patient)including a leak or bias flow or by determining patient flow Q1_(patient) that factors out leak or bias flow, can be used in thepresent invention. In general, patient flow Q_(patient) or Q1_(patient), both of which are also referred to as estimated patientflow, can be used interchangeably given stable leak conditions, exceptthat patient flow Q1 _(patient) does not use any leak or bias flowestimation, while patient flow Q_(patient) does. As will be noted below,there is at least one instance where this difference must be taken intoaccount. See, e.g., Trigger #6 and the corresponding cycling technique.

Ventilator system 30 also uses the average leak rate Q_(leak(average))for various purposes discussed below, such as for providing a bias flowto the patient to compensate for leaks in the system. The average leakrate Q_(leak(average)), which is the leak rate in liters per minute fora breathing cycle T_(breath), is determined by first calculating theleak volume V_(loss) during the breathing cycle. V_(loss) is determinedas: $\begin{matrix}{{V_{loss} = {T_{s}{\sum\limits_{i = 0}^{n}\left( {Q_{primary} + Q_{secondary} - Q_{exhaust}} \right)}}},} & (9)\end{matrix}$

where T_(s) is the sampling period, i=0 is the first sample instance inthe breathing cycle, and n is the last instance in the breathing cycle.The average leak rate Q_(leak(average)) is then determined asQ_(leak(average))=(V_(loss)/T_(breath))*0.06. The multiplier 0.06 is aconversion factor that is selected because V_(loss) in the exemplaryembodiment of the present invention is determined in milliliters andT_(breath) is determined in second, while Q_(leak(average)) is expressedin liters per minute. Those skilled in the art can appreciate that otherconversion factors or no conversion factors may be used depending on theunits being used. In the above example, the average leak rate isdetermined for each breath. It is to be understood, however, that anaverage leak rate can be calculated for more than one breathing cycle.

2. Bias Flow

In a preferred exemplary embodiment, the ventilator system of thepresent invention provides a bias flow to the patient so that a constantflow of gas is passing through the patient circuit. The magnitude of thebias flow is dependent upon the characteristics of the patient beingventilated, such as the patient's lung capacity, and the average leakrate Q_(leak(average)). More specifically, for an adult patient, thebias flow is determined as the average leak flow in liters per minuteplus a constant base rate, preferably 5 liters per minute. For apediatric patient, the bias flow is determined as average leak flow plusa constant 3 liters per minute. In summary:

Adult: Bias Flow=Q_(leak(average))+5 lpm

Pediatric: Bias Flow=Q_(leak(average))+3 lpm.

In a preferred embodiment of the present invention, the value for thebias flow is recalculated for each breathing cycle and the new bias flowvalue is used to provide the bias flow in the next breathing cycle. Inaddition, for safety purposes, the average leak rate is bounded by amaximum value of 60 lpm, so that the maximum bias flow that can beprovided to an adult, regardless of the actual average leak rate is 65lpm and the maximum bias flow for a pediatric patient is 63 lpm.

Those skilled in the art can appreciate that the constant base rateadded to the average leak rate need not be specifically limited to 5 and3 for adult and pediatric patients, respectively. On the contrary, othervalues for the constant base rate can be selected depending on the sizeof the patient, for example, or other considerations, such as thecondition of the patient. In addition, the constant leak rate can beeliminated or other selections, in addition to or in place of adult andpediatric, can be provided to the ventilator operator can moreaccurately match the requirements of the patient with the appropriatevalue for the constant base rate. In addition, the maximum average biasflow need not be specifically set to 60 lpm, rather other maximum valuesin this general range are contemplated by the present invention.Furthermore, the bias flow need not be recalculated every breathingcycle, but may be calculated more frequently or less frequently, so longas the effectiveness of the ventilator system is not compromised.

3. Triggering

The triggering process of the present invention is discussed below withreference to FIGS. 3-4. As shown in FIG. 3, the triggering process ofthe present invention effectively divides the exhalation phase T_(exp)into the following four segments: 1) a restricted segment 100, 2) anactive exhalation segment 102, 3) a non-active exhalation segment 104,and 4) a quiet exhalation segment 106. Establishing these segments inthe expiratory cycle of the patient's breathing cycle is done to allowactivation of one or more triggering mechanisms during each processingcycle based on the conditions that occur in each segment of theexhalation phase. FIG. 4 is a flowchart illustrating, in general, anexemplary triggering process implemented by the ventilator system. In apreferred embodiment of the present invention, all allowable triggermechanisms, i.e., Triggers #1-#7, are armed so that they can be testedfor activation during each processing cycle, and once a trigger is armedor enabled, it remains armed and awaiting activation for that processingcycle, and for the rest of the exhalation phase so long as theconditions needed to enable or arm the triggering mechanism remainsatisfied.

Arming different triggering mechanisms during different phases of theexpiratory phase causes the effective sensitivity of the ventilatorsystem triggering mechanism to increase the further the patient goesinto the expiratory phase of the breathing cycle. For example, it isunlikely that immediately after transitioning into the expiratory phase,the patient will attempt an inspiration. Therefore, the ventilatorsystem's sensitivity to a spontaneous inspiration at that time need notbe very high. On the other hand, when the patient nears the end of theexpiratory phase, it is very likely that that patient will soon beattempting to make a spontaneous inspiratory effort. Therefore, theventilator system's sensitivity to detecting an inspiratory effortshould be maximized at the end of the expiratory phase to detect theinspiratory effort reliably while minimizing the effort required by thepatient to trigger the ventilator. FIG. 3 illustrates when the varioustriggers effectively become active during the various stages of theexpiratory cycle. The operational definition of each segment of theexhalation phase T_(exp) is discussed in turn below.

In the present invention, there are four basic types of triggermechanisms: a pressure trigger, a flow trigger, a volume trigger, and aneffort trigger. Trigger #1 is a pressure trigger that tests the patientpressure against a threshold pressure with a relatively high sensitivitylevel. Trigger #2 is a flow trigger that tests the patient flow againsta threshold flow. Triggers #3 and #4 are volume triggers that test avolume against a threshold volume. More specifically, Trigger #3 teststhe estimated patient inhaled volume over the course of an increasingpatient flow pattern against a threshold volume, and Trigger #4 teststhe inhaled patient volume over 50 ms against a threshold volume. Assuch Trigger #3 is more of a long term trigger, looking as longer termtrends in the patient, and Trigger #4 is more of short term trigger,looking at the patient's more immediate volume. The effort triggerrefers to a determination of the patient's inspiratory effort based on across correlation of patient pressure and patient flow, which iscompared to a threshold effort level to determine whether to trigger theventilator. Triggers #5, #6 and #7 are effort based triggers, withTriggers #5 and #7 being longer term triggers, looking at patient effortover longer period of time, than Trigger #6, which is a short termeffort trigger that looks at the amount of inspiratory effort thepatient is exerting over a short period of time.

The following parameters are used in implementing the triggeringalgorithm of the present invention, and are constrained as definedbelow.

CompFlow: −1 lpm or −5% of the average leak Q_(leak(average)), whicheveris algebraically smaller.

MinFloT2: −4 lpm if Q_(leak(average)) is less than 30 lpm, 0 lpmotherwise.

Beta: 0.1 ml if expiration time≦inspiration time, otherwise:

Beta=0.2 ml if Bias Flow<15 lpm; or

Beta=0.5 ml if Bias Flow≧15 lpm and<30 lpm; or

Beta=1 ml if Bias Flow≧30 lpm.

MinStTime: 100 ms if Q_(leak)<30 lpm or 200 ms if Q_(leak)≧30 lpm.

MinTimeT3: 150 ms if Q_(leak)<30 lpm or 250 ms if Q_(leak)≧30 lpm.

MinVolT3: 3 ml if Q_(leak)<60 lpm or 4 ml if Q_(leak)≧60 lpm.

Sigma:

Pediatrics: Sigma=1 cmH₂O*lpm.

Adult:

Sigma=1 cmH₂O*lpm if Q_(leak)<30 lpm, and

Sigma=3 cmH₂O*lpm if Q_(leak)≧30 lpm.

It should be understood that the values for the above parameters aredependent on the physical characteristics of the components used inventilator system 30. For example, the configuration of the exhaust flowcontrol element, i.e., the exhaust valve, and/or the tubing used in thepatient circuit can affect the specific values of these parameters.Thus, the present invention is not intended to be limited to thespecific values of the parameters set forth above, but can encompass arange of values so long as the ventilator system functions in accordancewith the principles of the present invention. In addition, the presentinvention contemplates that the values for these parameters can beadaptive, to maximize the operation of the ventilator, or they can bemanually controllable to allow the ventilator operator a great degree offlexibility in setting up the ventilator to suit the needs of any givenpatient.

Referring now to FIGS. 3 and 4, restricted segment 100 is a shortduration at the outset of the exhalation phase, which begins at a cyclepoint 98, during active exhalation segment 102. In restricted segment100, no triggers from expiation to inspiration are permitted. In apreferred embodiment of the present invention, the restricted segment isset to 200 ms. It is to be understood, however, that the duration of therestricted segment can vary over a range of values around this generaltime frame. The 200 ms time frame is selected in the present inventionbecause for the most part, a human being is physically unable to returnto the inspiratory cycle within this short of a time frame aftercommencing the expiratory cycle.

In step 108, exhalation begins, and in step 110, processor 46 determineswhether 200 ms have elapsed since the start of exhalation. If not, theexpiratory flow is still within the restricted segment, and step 110 isrepeated until the duration of the restricted segment has elapsed, asindicated by feedback loop 112. If 200 ms have elapsed since the startof exhalation, the processor continues to steps 114, 116, 118, and 120.

After the 200 ms delay, i.e., restricted segment 100, the patient entersthe unrestricted portion of active exhalation segment 102. Activeexhalation segment 102 is defined as the interval, following the end ofthe restricted segment, during which the flow through exhaust flowcontrol element 64 Q_(exhaust) exceeds the total delivered flowQ_(total) by more than 5 liters per minute (lpm). This is the intervalof the expiratory phase that the patient is actively expelling gas fromthe lungs. To be conservative, leak Q_(leak) is not considered indetermining if the patient is in the active exhalation segment.

In active exhalation segment 102, Trigger #1, which is a backup pressuretrigger is armed, i.e., made ready for activation should the properconditions occur, as indicated in step 114. Trigger #1 is a conventionalpressure trigger with a sensitivity of 3 cmH₂O, so that if the patientpressure P_(patient) is less than the set PEEP level by 3 cmH₂O or more,the ventilator triggers. It can be appreciated that Trigger #1 requiresa relatively large amount of patient effort in order to decrease thepatient pressure 3 cmH₂O to prevent false triggers. Such false triggersare likely to occur if the ventilator's trigger sensitively is too high,because the active exhalation segment represents a portion of theexpiratory phase where patient flow is relatively unstable, i.e., flowrates can differ greatly over relatively short periods of time.

In step 116, processor 46 determines if the PEEP is to set to zero. Ifso, Trigger #2, which is a conventional flow backup trigger of 2 lpm isarmed in step 122. According to Trigger #2, if the patient flowQ_(patient) is positive and, if the patient flow is more than 2 lpm, theventilator will trigger. It can be appreciated that even though Trigger#2 is first checked during active exhalation segment 102, Trigger #2cannot be used to trigger the ventilator until the patient is innon-active exhalation segment 104, because the patient flow will not bepositive during the active exhalation segment. Those skilled in the artcan further appreciate that this trigger is optional depending on theoperating characteristics of the ventilator being used to implement theteachings of the present invention.

According to the present invention, a patient is in non-activeexhalation segment 104 when the patient flow Q_(patient) exceeds acertain minimum threshold CompFlow, which, as noted above, is −1 lpm or−5% of the average leak Q_(leak(average)), whichever is algebraicallysmaller. The patient is also considered to be in non-active exhalationsegment 104 when (1) the exhaust flow Q_(exhaust) is less than thedelivered flow Q_(total)+5 lpm and (2) the volume differential, a 100 msVolume Differential(n), (see equation (7)) has stabilized for at least acertain amount of time. The Volume Differential(n) is considered to bestable if, for example, consecutive absolute values of VolumeDifferential(n) are less than 0.1 ml for an amount of time MinStTime,which as noted above, is 100 ms if Q_(leak) is less than 30 lpm, or 200ms if Q_(leak) is greater than or equal to 30 lpm.

In step 118, the ventilator system determines if the patient flowQ_(patient) is positive. If so, Trigger #3 is armed in step 124. If not,the system re-checks every processing cycle, as indicated by feedbackloop 119. According to Trigger #3, if the patient flow starts to rise,its initial value is saved as a reference flow Q_(ref). Then, thedifference between the current flow Q_(patient(current)) and thereference flow Q_(ref) is accumulated during each processing cycle n,which in an exemplary embodiment is 5 ms.

Accumulating this difference (Q_(patient(current))−Q_(ref)) continues insubsequent processing cycles, so long as all three of the followingconditions are met: 1) the patient flow Q_(patient) is greater than thereference flow Q_(ref), 2) the Volume Differential(n) is greater than 0,and 3) the amount by which the current patient flow exceeds a priorpatient flow is greater than a certain amount, i.e., the slope of theflow increase is at least a certain value. In a preferred embodiment ofthe present invention, this third condition is determined by comparingthe current patient flow Q_(patient)(n) to the sum of a (1) time-delayedpatient flow Q_(patient(delayed))(n) and a (2) constant flow rate, suchas 0.5 lpm, i.e., Q_(patient)(n)>Q_(patient(delayed))(n)+5 lpm, where nis one processing cycle of 5 ms. In this embodiment,Q_(patient(delayed))(n)=0.2Q_(patient)(n)+0.8Q_(patient(delayed))(n−1).If any one of these conditions is not met, Trigger 3# is reset, whichcauses Q_(ref) to reset and the accumulated values to be reset to zero.

The accumulated values of the difference (Q_(patient(current))−Q_(ref))correspond to the patient volume because a difference(Q_(patient(current))−Q_(ref)) is determined every 5 ms, i.e., eachprocessing cycle. To obtain the volume, the difference is multiplied bytime, such as 5 ms for one processing cycle. If the running sum ofpatient volume exceeds a threshold volume, such as 7 ml in a preferredembodiment of the present invention, over any length of time, a triggeris declared. As noted above, the patient running volume sum is reset ifany of the above three conditions is breached.

In step 120, the Volume Differential(n) is monitored to determine if ithas remained stable for a period of time MinTimeT3, which, as notedabove, is 150 ms if the leak Q_(leak) is less than 30 lpm or 250 ms ifQ_(leak) is greater than or equal to 30 lpm. The Volume Differential(n)is considered to be stable if, for example, consecutive absolute valuesof Volume Differential(n) are less than 0.1 ml for an amount of timeMinTimeT3. If so, Trigger #4 is armed in step 126. If not, the systemre-checks every processing cycle, as indicated by feedback loop 121.According to Trigger #4, if the 50 ms Volume Differential(n) exceedsMinVolT3, which, as noted above, is 3 ml if Q_(leak) is less than 60 lpmor 4 ml if Q_(leak) is greater than or equal to 60 lpm, then a triggeris declared.

If the patient is still in non-active exhalation segment 104 asdetermined according to the criteria set forth above, i.e., if thepatient flow is level or the stability conditions are satisfied, asearch to determine whether a “trigger process” is conducted in step128. This is only done if a valid trigger process is not indicated ashaving been detected, i.e., a trigger process flag is false. A triggerprocess corresponds to a situation where the patient flow is increasingand the proximal pressure is decreasing.

According to an exemplary embodiment of the present invention, detectinga valid trigger process requires that all three of the followingconditions during a current processing cycle n be satisfied:

1) Q_(patient)(n−10)>CompFlow;

2) P_(patient)(n−10)≧1.2 * set PEEP or 0.5 H₂O (whichever is bigger);and

3) Q_(patient)(n)>Q_(patient)(n−10), and[P_(patient)(n−10)−P_(patient)(n)]>0.3 cmH₂O or P_(patient)(n)<0.8 setPEEP.

The first condition requires that the patient flow 50 ms prior to thecurrent patient flow is greater than CompFlow. The second conditionrequires that the patient pressure 50 ms prior to the current patientpressure is less than or equal to 120% of the set PEEP level or 0.5 H₂O,whichever is bigger. The third condition requires that the patient flowbe increasing, i.e., Q_(patient)(n)>Q_(patient)(n−10), and that thepatient pressure be decreasing, i.e., P_(patient)(n−10)−P_(patient)(n)or less than 80% of the set PEEP. If the three conditions set forthabove are satisfied, a trigger process is declared, i.e., the triggerprocess flag is set to true. In which case, the patient flow at thestart of the trigger process Q_(patient)(n−10) is set as a referenceflow Q_(ref) and the patient pressure at the start of the triggerprocess P_(patient)(n−10) is set as a reference pressure P_(ref). If thethree conditions set forth above are not satisfied, a trigger process isnot declared, i.e., the trigger process flag is set to false, and thesystem re-checks for a trigger process every processing cycle, asindicated by feedback loop 130.

If a valid trigger process is indicated as having already been detected,i.e., a trigger process flag is true when step 128 in the currentprocessing cycle is reached, then in the current processing cycle, thecurrent patient flow Q_(patient)(n) is compared to the reference flowQ_(ref) and the current patient pressure P_(patient)(n) is compared tothe reference pressure P_(ref). If Q_(patient)(n)≧Q_(ref) andP_(patient)(n)≦P_(ref), then a test for a trigger, such as Triggers #5,#6 and #7, can be made.

In step 132, which is reached as long as the trigger process remainstrue, an effort based Trigger #5 is armed. According to Trigger #5, theproduct of a patient flow difference (Q_(patient)−Q_(ref)) and apressure difference (P_(ref)−P_(patient)) is calculated and comparedagainst a leak-based threshold to determine a valid trigger. In apreferred embodiment, this threshold is 1.0 cmH₂O*ml/s. So that if(Q_(patient)−Q_(ref)) * (P_(ref)−P_(patient)) is greater than 1.0cmH₂O*ml/s, a trigger is declared.

The following conditions must be met in order for checking this trigger:

1) P_(patient)≧CompFlow;

2) P_(patient)<0.3 cmH₂O or 1.2*set PEEP (120% of set PEEP) whichever isbigger; and

3a) Q_(patient)>MinFloT2, or

3b) the volume based stability condition for establishing that thepatient is in the non-active exhalation segment has been met.

In step 134, the ventilation system checks to determine whether thepatient is in quiet exhalation segment 106 by checking the followingconditions:

1) Q_(exhaust)<Q_(total)+5 lpm;

2) Q_(patient)>CompFlow;

3) Volume Differential has remained stable for at least MinStTime; and

4) The sum of the absolute values of Volume Differentials over 50 ms isless than Beta.

If these conditions are not met, the system re-checks for a quietsegment every processing cycle, as indicated by feedback loop 136. If,however, all of these conditions are met, then the patient is deemed tobe in quiet exhalation segment 106 and the system proceeds to step 138.

In addition to the triggering options already armed as discussed above,two other effort based triggering options Triggers #6 and #7 are armedin step 138. These two effort based triggering algorithms are based onthe comparison of a measure of the estimated patient effort using twodifferent patient flow estimation methods and time scales so that ashort term effort trigger (Trigger #6) and a longer term trend basedeffort trigger (Trigger #7) are provided in step 138. More specifically,in step 138, if the differential patient effort over 100 ms exceeds aleak based threshold (short term effort Trigger #6), then a trigger isdeclared. Also in step 138, if the accumulated patient effort, which isthe time integral of the product of patient flow deviation from areference and patient pressure deviation from a reference (longer termeffort Trigger #7) exceed a leak based effort threshold, then a triggeris declared.

According to Trigger #6, when the quiet exhalation has been established(step 134) and the trigger process is holding true, the EstimatedPatient Effort (EPE) over a 100 ms window is compared against a constantthreshold (sigma). If EPE equals or exceeds sigma, a trigger isdeclared. In one embodiment of the present invention, the EPE isdetermined as the sum of the products of a patient flow differenceΔQ_(patient) and a filtered, i.e., delayed, pressure difference(FΔP_(patient)) over a 100 ms interval.

The purpose of the filtering is to delay the patient pressure used inthe pressure difference function, because, as noted above, at the onsetof inspiration there is a small delay between the onset of the pressuredrop and the rise in patient flow. This delay allows the current patientflow to be multiplied by a patient pressure difference that isdetermined using a patient pressure that corresponds to the currentpatient flow. The pressure difference is measured between the pressurereference P_(ref) at the start of the trigger window (see step 128) andthe current patient pressure P_(patient).

EPE is determined every processing cycle, e.g., every 5 ms. At controlcycle n: $\begin{matrix}{{{{EPE}(n)} = {\sum\limits_{n - 19}^{n}{F\quad \Delta \quad {P_{patient}(n)}*\Delta \quad {Q_{patient}(n)}}}},} & (10)\end{matrix}$

where,

ΔP _(patient)(n)=P _(ref) −P _(patient)(n),  (11)

FΔP _(patient)(n)=0.33ΔP _(patient)(n)+0.67 FΔP _(patient)(n−1),and  (12)

ΔQ _(patient)(n)=Q _(patient)(n)−Q _(ref).  (13)

Q_(patient)(n) is the current patient flow and Q_(ref) is the patientflow at the start of the triggering window, see step 128. Equation (12)represents a digital filter of pressure differential ΔP(n), which asnoted above, is provided to introduce a delay in patient pressurecomponents of the estimated patient effort calculation. For purposes ofTrigger #6, Q_(patient) is set to zero (Q_(patient)=0) if either (a)Q_(patient)<0 or (b) P_(patient)<3 ml/s and FΔP_(patient)<0.2 cmH₂O.

If patient flow is determined as discussed above with respect toequation (8), i.e., the estimated patient flow Q1 _(patient) factors outany stable bias or leak flow, or if there is no leak or bias flow, e.g.,the patient flow is measured directly at the patient, then Q_(ref) inequation (12) is effectively zero. That is, it is assumed that thereference flow at the start of the trigger window is zero, and thesystem need only look for an increase from this baseline or zero value.In which case, EPE is determined as: $\begin{matrix}{{{{EPE}(n)} = {\sum\limits_{n - 19}^{n}{F\quad \Delta \quad {P_{patient}(n)}*{{Q1}_{patient}(n)}}}},} & (14)\end{matrix}$

where Q1 _(patient(prior)) from equation (8) is set equal to zero at thestart of the trigger window. As noted above, if EPE(n) determined usingeither equation (10) or (14) is greater than sigma, a trigger isdeclared.

According to Trigger #7, under the quiet exhalation condition, when atrigger process is established as true, as long as the trigger processstays true, the product of (Q_(patient)−Q_(ref))*(P_(ref)−P_(patient))is accumulated from each processing cycle. If this running sum equals orexceeds 1.5 cmH₂O*ml/s a trigger is declared. In this way, a relativelylong term patient effort trend is monitored for a triggering event.

4. Cycling

The present invention contemplates using one of following two cyclingtechniques to transition from the inspiratory phase to the expiratoryphase of the breathing cycle: 1) an effort based technique that is basedon the combination of patient flow and patient pressure, and 2) anadaptive, flow based technique where cycling is determined by comparingthe current patient flow to a threshold flow. Essentially, the functionof the cycling event is to cause the ventilator to allow patient flow tobe expelled from the lungs. This is accomplished in ventilator system30, for example, by opening or increasing the degree of opening ofexhaust flow control element 64 at the time the patient begins theexpiratory effort.

The effort based cycling technique was discussed briefly above and isessentially the same as Triggers #5, #6, and #7, except that a cycleevent is indicated by an algebraic decrease in flow followed by anincrease in pressure. Thus, the patient flow is delayed by a small timefactor before the product of the patient flow and patient pressuredifference over a certain time frame is determined and compared to athreshold expiratory effort level.

More specifically, cycling in a manner similar to Trigger #5 discussedabove involves determining a patient flow difference(Q_(ref)−Q_(patient)) and patient pressure difference(P_(patient)−P_(ref)). The product of these differences(Q_(ref)−Q_(patient))*(P_(patient)−P_(ref)) is determined and comparedto a threshold to determine a valid cycle. The threshold value ispreferably determined empirically based on clinical trials and can bemade adaptive to match the changing conditions of the patient.

Cycling in a manner similar to Trigger #6 discussed above involvesdetermining a patient pressure difference (P_(patient)−P_(ref),) similarto that done in equation (11), where P_(patient) is the current patientpressure and P_(ref) is a reference patient pressure determined at astart of a cycling window. In addition, a patient flow difference(Q_(ref)−Q_(patient)) similar to that done in equation (13) isdetermined, wherein Q_(patient) is the current patient flow, and Q_(ref)is a reference patient flow determined at the start of the cyclingwindow. Of course, Q_(ref) is only used if the determination of patientflow does not already automatically eliminate any bias flow or there isa leak or bias flow that should be compensated for.

It should be noted that for cycling purposes, the system looks for apressure increase from the reference pressure set at the start of thecycling window. Therefore, P_(ref) is subtracted from P_(patient),rather than subtracting P_(patient) from P_(ref) as done in Equation(11), to ensure that this patient pressure difference is indicative of apressure increase. The system also looks for a flow decrease from thereference flow set at the start of the cycling window, i.e., an increasein expiratory flow from the patient. Therefore, Q_(patient) issubtracted from Q_(ref), rather than subtracting Q_(ref) fromQ_(patient), as done in equation (13).

As noted above, for cycling/expiratory effort determination purposes,the present invention delays the patient flow and determines a productof the current patient pressure difference and the delayed patient flowas the patient's expiratory effort. This expiratory effort, which ispreferably determined every processing cycle (i.e., every 5 ms), issummed over a very short time interval, such as 100 ms. Cycling fromproviding the inspiratory flow to allowing an expiratory flow ofbreathing gas from the exhaust assembly is initiated if the sum of thepatient's expiratory efforts over this short time interval exceed athreshold. The value of this threshold can be determined empirically andcan be made adaptive to match the changing conditions of the patient.

Cycling in a manner similar to Trigger #7 discussed above, which looksat a longer term expiratory effort trend, involves determining a patientflow difference (Q_(ref)−Q_(patient)), where Q_(patient) is the currentpatient flow and Q_(ref) is a reference patient flow determined at thestart of the cycling window. It should be again noted that for cyclingpurposes, the system looks for a flow decrease, i.e., increasingexpiratory flow from the patient, from the reference flow set at thestart of the cycling window. Therefore, P_(patient) is subtracted fromQ_(ref), rather than subtracting Q_(ref) from Q_(patient), to ensurethat this patient flow difference is indicative of increasing expiratoryflow.

The system further determines a patient pressure difference(P_(patient)−P_(ref)), where P_(patient) is the current patient pressurefrom the first pressure signal and P_(ref) is a reference patientpressure determined at the start of the cycling window, again lookingfor an increase in patient pressure above the reference value determinedas the start of the cycling window. The patient's expiratory effort isdetermined as a product of the patient flow difference and the patientpressure difference. As done with Trigger #7, the system continues tosum these patient expiratory efforts over at time interval and initiatesa cycle if the sum of the patient's expiratory efforts over the timeinterval exceed a threshold. This threshold is also determinedempirically, and can be made adaptive.

Under the adaptive, flow based technique, the cycle threshold flow (CTF)is set as a percentage of the peak flow for that breath. In a preferredembodiment of the present invention, the CTF is initially set at 35% ofthe peak inspiratory flow. The patient flow during the inspiratory phaseis monitored, and once this flow falls below the cycle threshold flow,the ventilator cycles. The present inventors recognized that the CTFlevel cannot be fixed at this level, because the patient is unlikely toalways begin exhaling when their flow falls to 35% of their peak flowfor that breath. Accordingly, the present inventors developed a processto dynamically adjust the CTF.

To allow the CTF to adapt so that the ventilator cycles more closely insynchronization with the patient's expiratory effort, the presentinvention monitors the patient pressure P_(patient) at the end portionof the inspiratory phase and the patient flow Q_(patient) at thebeginning portion of the expiratory phase. If the patient pressurebegins to increase before the patient flow reaches the CTF, thisindicates that the patient has begun exhalation and the exhaust flowcontrol element has not yet been opened to permit the patient to exhalefreely. In other words, the ventilator cycled too late, i.e., after thepatient began exhaling. On the other hand, if the ventilator has cycledand there is substantially no patient flow from the patient, thisindicates that the patient has not yet begun to exhale even though theexhaust flow control element has been opened. In other words, theventilator cycled too early, i.e., before the patient began exhaling.The present invention monitors the patient pressure and flow todetermine if either of these errors in synchronization have occurred,and adjusts the CTF in the next breath to account for the cyclingsynchronization error in the previous breath.

To determine whether the ventilator cycled too late, the patientpressure at the end of the inspiratory phase P_(patient(insp end))(k)and the patient pressure at a time, such as 100 ms, before the end ofthe inspiratory phase P_(patient(insp end))(k−100) are obtained. IfP_(patient(insp end))(k)>P_(patient(insp end))(k−100)+0.5 cmH₂O, theventilator cycled too late. In a preferred embodiment of the presentinvention, when this occurs, the CTF is increased a predeterminedamount, for example 20% so that the ventilator cycles sooner in the nextbreathing cycle.

To determine whether the ventilator cycled too early, the volume offluid from the patient during the initial portion of the expiratoryphase V_(exp)(k) is compared to a percentage, such as 25%, of a volumeof fluid inspired by the patient during a similar time period in thepreceding inspiratory phase V_(insp)th. If the ventilator cycled tooearly, the volume of fluid from the patient during the initial portionof the expiratory phase will be less than the volume of fluid inspiredby the patient during a similar time period in the preceding inspiratoryphase.

In a preferred embodiment of the present invention, the volume of fluidfrom the patient during the first 300 ms of the expiratory phaseV_(exp)(k) is determined as: $\begin{matrix}{{V_{\exp}(k)} = {\int_{0}^{300{ms}}{{Q_{patient}(t)}{{t}.}}}} & (13)\end{matrix}$

Twenty-five percent of prorated inhaled patient volume estimate for 300ms of the inhalation period V_(insp)th is determined as: $\begin{matrix}{{{V_{insp}{th}} = {\left( \frac{75}{T_{insp}} \right)\left\{ {{\int_{0}^{T_{insp}}{\left( {Q_{primary} + Q_{secondary} - Q_{exhaust}} \right){t}}} - {T_{insp}*{LF}*\sqrt{P_{{patient}{({{insp}\quad {end}})}}}}} \right\}}},} & (14)\end{matrix}$

where P_(patient(insp end)) is the end inspiratory pressure, which isassumed to correspond to the average patient pressure over the length ofthe inspiratory phase. It is to be understood, however, that the averagepatient pressure over the length of the inspiratory phase can bedetermined directly so that an approximation need not be used. In apreferred embodiment of the present invention the value for V_(insp)this bounded as follows.

adult: 40 ml≦V_(insp)th≦200 ml, and

pediatric: 10 ml≦V_(insp)th≦40 ml.

If the ventilator did not cycle too late, V_(exp)(k) is compared toV_(insp)th. It should be noted that V_(exp)(k) and V_(insp)th will haveopposite signs, because V_(exp)(k) represents a volume exhaled andV_(insp)th represents a volume inhaled. If the absolute value ofV_(exp)(k) is less than the absolute value of V_(insp)th, then thepatient is not trying to exhale, i.e., the ventilator cycled too soon.In which case, the CTF is decreased a predetermined amount, for example10%, so that the ventilator cycles later in the next breathing cycle. Ifthe ventilator did not cycle too late, and if the absolute value ofV_(exp)(k) is not less than the absolute value of V_(insp)th, then theCTF remains unchanged.

It is to be understood that the amount and rates at which the CTF isincreased and decreased can be varied depending on how aggressively theventilator should attempt to correct for cycling synchronization errors.It is preferable, however, that the CTF be bounded, for example, between3% and 60% of the peak flow for that breath. 300 ms is used for the timeperiod of the window at the beginning of the exhalation phase duringwhich the patient's flow/volume is monitored is selected because it islikely that a patient who is attempting to exhale will producemeasurable results within 300 ms of beginning to exhale.

It is to be understood, that cycling can also be accomplished usingconventional cycling techniques. However, under the cycling option ofthe present invention, the cycling criteria are adaptively altered basedon conditions obtained from the previous breath for maximum patientcomfort.

The triggering and cycling parameters discussed above, such as thespecific threshold levels and timings, are selected so that the abovedescribed triggering and cycling techniques perform effectively whenimplemented on an Esprit Ventilator across all possible lung parameters(resistance, compliance) for each patient type (adult, pediatric) andinherent variability and measurement uncertainties (noise, etc.). It isto be understood, however, that the present invention contemplatesadaptively changing the algorithmic parameters of the present inventionduring the breathing cycle based on an estimation of desirable endpoints and optimization strategy to achieve a desired goal or goals. Forexample, triggering performance, e.g., thresholds, may be dynamicallyoptimized based on minimizing the work of breathing required to triggera breath on one hand and minimizing the ventilator autocycling (falsetriggering) on the other. Thus, for this example, the triggeringcriterion may be adjusted breath by breath using any conventionaltechnique, such as dynamical programming, neural networks, fuzzy logic,etc., while signs of autocycling are being estimated or monitored. Theadjustment of triggering criterion would change direction or weightingas the measure of autocycling approaches or exceeds a minimum threshold.

Autocyclying may be detected, for example, based on analysis of possiblerange of lung mechanics, rate and speed of change of pressure and flowmeasurements, delivered tidal volume versus exhaled volume, etc. Onefeasible method is to observe the rate of change of tidal volume forsimilar breath settings. For example, under pressure-controlled breathdelivery, the faster the ventilator autocycles, the smaller thedelivered tidal volume would become for consecutive breaths, because thepatient does not get enough time to exhale and the same pressure levelwill be reached with a smaller inspiratory volume.

5. Leak Rate Error Correction and Display

It can be appreciated that in the ventilator system of the presentinvention, some situations may occur where the leak rate Q_(leak) cannotbe accurately determined. To account for these contingencies, thepresent invention implements a process for checking whether the leakrate determined during the current breathing cycle and/or processingcycle is valid, and for ensuring proper operation of the ventilatorsystem even if the leak rate during a processing cycle is not valid. Inparticular, the following logic algorithm is implemented to reset orcontrol the leak factor LF and the cycle threshold flow (CTF) when theactual leak rate is undetermined during a processing cycle.

According to this process, during each processing cycle, a check is madeto determine if the leak rate determined for that cycle is valid. With aconstant bias flow in the ventilation system, the leak rate is deemedinvalid if the exhaust flow Q_(exhaust) is significantly less than thebias flow. This can occur, for example, if the patient interface devicebecomes dislodged so that little flow is being exhausted through exhaustassembly 56. In a preferred embodiment of the present invention, thecurrent leak rate Q_(leak)(n) is also deemed invalid if (1) the exhaustflow Q_(exhaust) is less than 1 LPM during the expiratory phase of thebreathing cycle, or (2) a worst case estimated compliance factor exceedsextreme thresholds. In a preferred embodiment of the present invention,the first condition is not checked during the first 200 ms restrictedsegment of the exhalation phase.

The worst case estimated compliance factor (C_(WC)) is computed asfollows. During each inhalation phase, a worst case total gas volume(WCTV) delivered to the patient is computed assuming extreme leak rates(Q_(leak, max)) of 80 liters per minute (lpm) for pediatrics and 100 lpmfor adults. This first involves comparing the flow of gasses deliveredto the patient Q_(delivered) with the extreme leak rate Q_(leak, max)during each processing cycle (n) to determine a difference ΔQ(n)therebetween. In other words, ΔQ(n) is determined during each processingcycle (n) as follows:

ΔQ(n)=Q(n)_(delivered) −Q _(leak, max),  (15)

where Q(n)_(delivered) corresponds to the sum of all of the flows of gasdelivered to the patient, i.e.,Q(n)_(delivered)=Q_(primary)+Q_(secondary). If the delivered flowQ_(delivered) is less than Q_(leak, max,) then ΔQ(n) is set to zero. Theworst case total gas volume WCTV is then determined over the inhalationcycle by summing the differences ΔQ(n) between Q_(delivered) andQ_(leak, max) determined during each processing cycle over theinhalation period. That is, WCTV is determined as: $\begin{matrix}{{{WCTV} = {0.005*{\sum\limits_{n = 0}^{n = N}{\Delta \quad {Q(n)}}}}},} & (16)\end{matrix}$

where n is a processing cycle, i.e., a 5 ms time interval, and N is thetotal number of processing cycles in the inhalation period of thebreathing cycle.

The worst case estimated compliance factor C_(WC) is then determined as:

C _(WC) =WCTV/PIP,  (17)

wherein PIP=peak inspiratory pressure. At the end of every inhalation,if C_(WC) is greater than 150 ml/cmH₂O for pediatrics, or 300 ml/cmH₂Ofor adults, then the leak rate estimate is considered invalid. It is tobe understood that the threshold against which C_(WC) is compared can bevalues other than 150 ml/cmH₂O for pediatrics or 300 ml/cmH₂O for adultsand the present invention is not intended to be limited to theseparticular values.

A first flag (Leak Flag 1) and a second flag (Leak Flag 2) are initiallyset to true. Thereafter, the value of these flags is altered as setforth below, to control the leak factor LF and the cycle threshold flow(CTF) depending on the values of these flags. If, during a processingcycle, the above condition for an invalid leak rate is met, the twoflags are both set to false, i.e., Leak Flag 1=false, and Leak Flag2=false, meaning that the current leak rate determination should not beused by the processor. If, on the other hand, the above condition for aninvalid leak is not met, i.e., the leak rate is valid, Leak Flag 1 isset to true. Note that Leak Flag 2 is not necessarily set to true atthis time.

The ventilator system then determines if the patient is in theexhalation phase. If so, and if the second flag (Leak Flag 2) is falseand the first flag (Leak Flag 1) is true, then the patient is consideredto be in the non-active exhalation segment for triggering purposes, andthe leak factor is set to zero. This allows the ventilator to properlytrigger even though the leak rate has not been valid for an entirebreath cycle. If the patient is in the exhalation phase and the secondflag (Leak Flag 2) is true, the leak rate error process continues, andthe second flag (Leak Flag 2) is not reset to at this time.

If the patient is in the inhalation phase, and the first and secondflags are false, then the CTF is set to 45% of the peak flow and theleak factor is set to zero. Furthermore, if the leak rate is determinedto be invalid, the bias flow reference is set to 20 lpm for adults andto 10 lpm for a pediatric patient. If the above conditions for aninvalid leak are not met, i.e., the leak rate is valid, the first flag(Leak Flag 1) is set to true and the second flag is also set to true.Because the second flag is only reset to true during the inspiratoryphase of the breathing cycle, it effectively forces the ventilatorsystem to collect one whole breathing cycle worth of valid leak ratedata before the ventilator system will use that leak flow data.

In an exemplary preferred embodiment of the present invention, theestimated leak flow Q_(leak) is displayed for each breath. If theestimated leak flow exceeds an alarm threshold, a high leak alarm isgenerated. This high leak alarm is preferably selectively set by theoperator.

The invention has been described above as being implemented in a digitalprocessor running at a certain operating speed. It is be understood,that this operating speed can be varied. In which case, it may benecessary to change certain constants used in the above calculations. Inaddition, it is to be understood that the present invention need not beimplemented in a digital processor. On the contrary, the entire system,or components of the system can be implemented in analog (continuous)form rather than in the digital (discrete) from discussed herein. Ofcourse, implementing all or parts of the system in an analog system mayrequire appropriate modification to the techniques discussed above.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims

What is claimed is:
 1. A system for providing a flow of gas to a patientcomprising: a pressure generating system adapted to provide a flow ofgas to a patient responsive to a control signal; a patient circuithaving a first end coupled to the pressure generating system and adaptedto communicate the flow of gas with an airway of a patient; a flowsensor adapted to measure the flow of gas in the patient circuit and tooutput a first flow signal indicative thereof; a pressure sensor adaptedto measure a pressure of the flow of gas in the patient circuit and tooutput a first pressure signal indicative thereof; an exhaust assemblyadapted to communicate gas from within the patient circuit to ambientatmosphere; and a controller that receives the first flow signal and thefirst pressure signal and outputs the control signal that controls theflow of gas delivered to the patient circuit by the pressure generatingsystem and, hence, the flow of gas at a patient's airway, wherein thecontroller detects onset of an inspiratory phase of a patient'sbreathing cycle for triggering an inspiratory flow of gas based on sucha patient's inspiratory effort, which is determined based on a productof a flow related parameter determined from the first flow signal and apressure related parameter determined from the first pressure signal. 2.The system according to claim 1, wherein the pressure generating systemincludes: a blower that receives a supply of gas from a gas source andprovides the flow of gas; a flow controller associated with the blowerto control a rate of the flow of gas responsive to the control signal.3. The system according to claim 2, wherein the flow controller is aflow restricting valve disposed in the patient circuit downstream of theblower that controls the rate of the flow of gas by restricting a flowof gas in the patient circuit responsive to the control signal.
 4. Thesystem according to claim 1, wherein the flow sensor is disposed in thefirst end of the patient circuit.
 5. The system according to claim 1,wherein the patient circuit is a two-limb circuit with a first limbhaving a first end operatively connected to the pressure generatingsystem and a second end, and a second limb having a first endoperatively connected to the exhaust assembly and a second end, whereinthe second ends of the first and the second limbs are located proximalto an airway of a patient during use of the system.
 6. The systemaccording to claim 5, wherein the exhaust assembly includes an exhaustflow controller to control a rate of the exhaust flow of gas from thepatient circuit responsive to an exhaust flow control signal provided bythe controller.
 7. The system according to claim 1, further comprising asecondary gas flow system that delivers a secondary flow of gas to thepatient circuit, wherein the secondary gas flow system includes: aconduit configured and arranged so as to communicate the secondary flowof gas from a source of the secondary flow of gas to the patientcircuit; and a second flow sensor adapted to measure the secondary flowof gas in the conduit and to output a second flow signal indicativethereof.
 8. The system according to claim 1, wherein the controllerestablishes a trigger lockout interval, which is a period of time duringeach expiratory phase of a breathing cycle in which triggering theinspiratory flow of gas is prevented, based on at least one of the firstflow signal and the first pressure signal.
 9. The system according toclaim 1, wherein the controller: determines a patient flow difference(Q_(patient)−Q_(ref)) as the flow related parameter, where Q_(patient)is a current patient flow from the first flow signal and Q_(ref) is areference patient flow determined from the first flow signal at a startof a trigger window, which is a period of time during which triggeringthe inspiratory flow of gas is permitted; determines a patient pressuredifference (P_(ref)−P_(patient)) as the pressure related parameter,where P_(patient) is a current patient pressure from the first pressuresignal and P_(ref) is a reference patient pressure determined from thefirst pressure signal at the start of the trigger window; determines thepatient's inspiratory effort as a product of the patient flow differenceand the patient pressure difference; and triggers the inspiratory flowof gas responsive to the patient's inspiratory effort exceeding athreshold.
 10. The system according to claim 1, wherein the controller:determines a patient flow difference (Q_(patient)−Q_(ref)) as the flowrelated parameter, where Q_(patient) is a current patient flow from thefirst flow signal and Q_(ref) is a reference patient flow determinedfrom the first flow signal at a start of a trigger window, which is aperiod of time during which triggering the inspiratory flow of gas ispermitted; determines a patient pressure difference(P_(ref)−P_(patient)) as the pressure related parameter, whereP_(patient) is a current patient pressure from the first pressure signaland P_(ref) is a reference patient pressure determined from the firstpressure signal at the start of the trigger window; determines thepatient's inspiratory effort as a product of the patient flow differenceand the patient pressure difference; sums the patient's inspiratoryefforts accumulated over a time interval; and triggers the inspiratoryflow of gas responsive to the sum of the patient's inspiratory effortsover the time interval exceeding a threshold.
 11. The system accordingto claim 10, wherein the time interval has fixed duration.
 12. Thesystem according to claim 1, wherein the controller: determines apatient pressure difference (P_(ref)−P_(patient)) as the pressurerelated parameter, where P_(patient) is a current patient pressure fromthe first pressure signal and P_(ref) is a reference patient pressuredetermined from the first pressure signal at a start of a triggerwindow; delays the patient pressure difference in time to determine adelayed patient pressure difference; determining a current patient flowfrom the first flow signal as the flow related parameter; determines aproduct of a current patient flow and the delayed patient pressuredifference as the patient's inspiratory effort; sums the patient'sinspiratory efforts accumulated over a time interval; and triggers theinspiratory flow of gas responsive to the sum of the patient'sinspiratory effort exceeding a threshold.
 13. The system according toclaim 1, wherein the controller detects onset of an expiratory phase ofa patient's breathing cycle for cycling from providing the inspiratoryflow of gas to allowing an expiratory flow of gas from the exhaustassembly based on such a patient's expiratory effort, which isdetermined based on both the first flow signal and the first pressuresignal.
 14. The system according to claim 13, wherein the controller:determines a patient flow difference (Q_(ref)−Q_(patient)), whereQ_(patient) is a current patient flow from the first flow signal andQ_(ref) is a reference patient flow determined from the first flowsignal at a start of a cycling window, which is a period of time duringwhich the expiratory flow of gas from the patient circuit is permitted;determines a patient pressure difference (P_(patient)−P_(ref)), whereP_(patient) is a current patient pressure from the first pressure signaland P_(ref) is a reference patient pressure determined from the firstpressure signal at the start of the cycling window; determines thepatient's expiratory effort as a product of the patient flow differenceand the patient pressure difference; and cycles from providing theinspiratory flow of gas to allowing an expiratory flow of gas from theexhaust assembly responsive to the patient's expiratory effort exceedinga threshold.
 15. The system according to claim 13, wherein thecontroller: determines a patient pressure difference(P_(patient)−P_(ref),), where P_(patient) is a current patient pressurefrom the first pressure signal and P_(ref) is a reference patientpressure determined from the first pressure signal at a start of acycling window, which is a period of time during which the expiratoryflow of gas from the patient circuit is permitted; delays a patient flowfrom the first flow signal to determine a delayed patient flow;determines a product of the patient pressure difference and the delayedpatient flow as the patient's expiratory effort; sums the patient'sexpiratory efforts accumulated over a time interval; and cycles fromproviding the inspiratory flow of gas to allowing an expiratory flow ofgas from the exhaust assembly responsive to the sum of the patient'sexpiratory effort exceeding a threshold.
 16. The system according toclaim 13, wherein the controller: determines a patient flow difference(Q_(ref)−Q_(patient)), where Q_(patient) is a current patient flow fromthe first flow signal and Q_(ref) is a reference patient flow determinedfrom the first flow signal at a start of a cycling window, which is aperiod of time during which the expiratory flow of gas from the patientcircuit is permitted; determines a patient pressure difference(P_(patient)−P_(ref)), where P_(patient) is a current patient pressurefrom the first pressure signal and P_(ref) is a reference patientpressure determined from the first pressure signal at the start of thecycling window; determines the patient's expiratory effort as a productof the patient flow difference and the patient pressure difference; sumsthe patient's expiratory efforts accumulated over a time interval; andcycles from providing the inspiratory flow of gas to allowing anexpiratory flow of gas from the exhaust assembly responsive to the sumof the patient's expiratory efforts over the time interval exceeding athreshold.
 17. The system according to claim 16, wherein the timeinterval has fixed duration.
 18. The system according to claim 1,wherein the controller cycles from providing the inspiratory flow of gasto allowing an expiratory flow of gas from the exhaust assembly bycomparing patient flow determined from the first flow signal against acycle threshold flow and cycles responsive to the patient flow fallingbelow the cycle threshold flow.
 19. The system according to claim 18,wherein the controller: monitors patient pressure, via the firstpressure signal, at an end portion of an inspiratory phase and monitorspatient flow, via the first flow signal, at a beginning portion of anexpiratory phase to determine whether the system cycled too early or toolate; and adjusts the cycle threshold flow for a next breathing cycleresponsive to a determination that the system cycled too early or toolate.
 20. A system for providing a flow of gas to a patient comprising:a pressure generating system adapted to provide a flow of gas to apatient responsive to a control signal; a patient circuit coupled to thepressure generating system and adapted to communicate the flow of gaswith an airway of a patient; a flow sensor adapted to measure the flowof gas in the patient circuit and to output a first flow signalindicative thereof; a pressure sensor adapted to measure a pressure ofthe flow of gas in the patient circuit and to output a first pressuresignal indicative thereof; an exhaust assembly adapted to communicategas from within the patient circuit to ambient atmosphere; and acontroller that receives the first flow signal and the first pressuresignal and outputs the control signal that controls the flow of gasdelivered to the patient circuit by the pressure generating system and,hence, the flow of gas at a patient's airway, wherein the controllerarms a plurality of triggering mechanisms over an expiratory phase of abreathing cycle to increase sensitivity to a patient initiated triggeras the expiratory phase of the breathing cycle progresses.
 21. A methodof providing a flow of gas to a patient comprising: generating a flow ofgas; providing the flow of gas to a patient via a patient circuit;controlling the flow of gas delivered to a patient responsive to acontrol signal; measuring the flow of gas in the patient circuit andoutputting a first flow signal indicative thereof; measuring a pressureof the flow of gas in the patient circuit and outputting a firstpressure signal indicative thereof; communicating gas from within thepatient circuit to ambient atmosphere; and detecting onset of aninspiratory phase of a patient's breathing cycle for triggering aninspiratory flow of gas based on such a patient's inspiratory effort,which is determined based on a product of a flow related parameterdetermined from the first flow signal and a pressure related parameterdetermined from the first pressure signal.
 22. The method according toclaim 21, wherein detecting the onset of the inspiratory phase includes:determining a patient flow difference (Q_(patient)−Q_(ref)) as the flowrelated parameter, where Q_(patient) is a current patient flow from thefirst flow signal and Q_(ref) is a reference patient flow determinedfrom the first flow signal at a start of a trigger window, which is aperiod of time during which the expiratory flow of gas is permitted;determining a patient pressure difference (P_(ref)−P_(patient)) as thepressure related parameter, where P_(patient) is a current patientpressure from the first pressure signal and P_(ref) is a referencepatient pressure determined from the first pressure signal at the startof the trigger window; and determining the patient's inspiratory effortas a product of the patient flow difference and the pressure difference.23. The method according to claim 22, further comprising triggering aninspiratory flow of gas responsive to the patient's inspiratory effortexceeding a threshold.
 24. The method according to claim 21, whereindetecting the onset of the inspiratory phase includes: determining apatient flow difference (Q_(patient)−Q_(ref)) as the flow relatedparameter, where Q_(patient) is a current patient flow from the firstflow signal and Q_(ref) is a reference patient flow determined from thefirst flow signal at a start of a trigger window, which is a period oftime during which the expiratory flow of gas is permitted; determining apatient pressure difference (P_(ref)−P_(patient)) as the pressurerelated parameter, where P_(patient) is a current patient pressure fromthe first pressure signal and P_(ref) is a reference patient pressuredetermined from the first pressure signal at a start of a triggerwindow; determining the patient's inspiratory effort as a product of apatient flow difference and the pressure difference; summing thepatient's inspiratory efforts accumulated over a time interval; andtriggering an inspiratory flow of gas responsive to the sum of thepatient's inspiratory efforts over the time interval exceeding athreshold.
 25. The method according to claim 24, wherein the timeinterval has a fixed duration.
 26. The method according to claim 21,wherein detecting the onset of the inspiratory phase includes:determining a patient pressure difference (P_(ref)−P_(patient)) as thepressure related parameter, where P_(patient) is a current patientpressure from the first pressure signal and P_(ref) is a referencepatient pressure determined from the first pressure signal at a start ofa trigger window, which is a period of time during which the expiratoryflow of gas is permitted; determining a delayed patient pressuredifference, where a length of the delay is selected so as to account foran inherent physiological delay between an onset of a pressure drop anda rise in patient flow occurring at a beginning of inspiration;determining a current patient flow from the first flow signal as theflow related parameter; determining a product of the current patientflow and the delayed patient pressure difference as the patient'sinspiratory effort; summing the patient's inspiratory effortsaccumulated over a time interval; and triggering an inspiratory flow ofgas responsive to the sum of the patient's inspiratory effort exceedinga threshold.
 27. The method according to claim 21, further comprisingdetecting onset of an expiratory phase of a patient's breathing cyclefor cycling from providing an inspiratory flow of gas to allowing anexpiratory flow of gas from the patient circuit based on such apatient's expiratory effort, wherein detecting the onset of anexpiratory phase is determined based on both the first flow signal andthe first pressure signal.
 28. The method according to claim 27, whereindetecting an onset of an expiratory phase includes: determining apatient flow difference (Q_(ref)−Q_(patient)), where Q_(patient) is acurrent patient flow from the first flow signal and Q_(ref) is areference patient flow determined from the first flow signal at a startof a cycling window, which is a period of time during which theexpiratory flow of gas from the patient circuit is permitted;determining a patient pressure difference (P_(patient)−P_(ref)), whereP_(patient) is a current patient pressure from the first pressure signaland P_(ref) is a reference patient pressure determined from the firstpressure signal at the start of the cycling window; determining thepatient's expiratory effort as a product of the patient flow differenceand the patient pressure difference; and cycling from providing aninspiratory flow of gas to allowing an expiratory flow of gas from thepatient circuit responsive to the patient's expiratory effort exceedinga threshold.
 29. The method according to claim 27, wherein detecting anonset of an expiratory phase includes: determining a patient pressuredifference (P_(patient)−P_(ref),), where P_(patient) is a currentpatient pressure from the first pressure signal and P_(ref) is areference patient pressure determined from the first pressure signal ata start of a cycling window, which is a period of time during which theexpiratory flow of gas from the patient circuit is permitted; delaying apatient flow from the first flow signal to determine a delayed patientflow; determining a product of the patient pressure difference and thedelayed patient flow as the patient's expiratory effort; summing thepatient's expiratory efforts accumulated over a time interval; andcycling from providing an inspiratory flow of gas to allowing anexpiratory flow of gas from the patient circuit responsive to the sum ofthe patient's expiratory effort exceeding a threshold.
 30. The methodaccording to claim 27, wherein detecting an onset of an expiratory phaseincludes: determining a patient flow difference (Q_(ref)−Q_(patient)),where Q_(patient) is a current patient flow from the first flow signaland Q_(ref) is a reference patient flow determined from the first flowsignal at a start of a cycling window, which is a period of time duringwhich the expiratory flow of gas from the patient circuit is permitted;determining a patient pressure difference (P_(patient)−P_(ref),), whereP_(patient) is a current patient pressure from the first pressure signaland P_(ref) is a reference patient pressure determined from the firstpressure signal at the start of the cycling window; determining thepatient's expiratory effort as a product of the patient flow differenceand the patient pressure difference; summing the patient's expiratoryefforts accumulated over a time interval; and cycling from providing aninspiratory flow of gas to allowing an expiratory flow of gas from thepatient circuit responsive to the sum of the patient's expiratoryefforts over the time interval exceeding a threshold.
 31. The methodaccording to claim 30, wherein the time interval has a fixed duration.32. The method according to claim 21, wherein the controller cycles fromproviding an inspiratory flow of gas to allowing an expiratory flow ofgas from the patient circuit by comparing patient flow determined fromthe first flow signal against a cycle threshold flow and cyclesresponsive to the patient flow falling below the cycle threshold flow.33. The method according to claim 32, further comprising: monitoringpatient pressure, via the first pressure signal, at an end portion of aninspiratory phase; monitoring patient flow, via the first flow signal,at a beginning portion of an expiratory phase; determining whethercycling occurred too late based on the patient pressure at the endportion of the inspiratory phase; determining whether cycling occurredtoo early based on the patient flow at the beginning portion of theexpiratory phase; and adjusting the cycle threshold flow for a nextbreathing cycle responsive to a determination that cycling occurred tooearly or too late.
 34. A method of providing a flow of gas to a patientcomprising: generating a flow of gas; providing the flow of gas to apatient via a patient circuit; controlling the flow of gas delivered toa patient responsive to a control signal; measuring the flow of in thepatient circuit and outputting a first flow signal indicative thereof;measuring a pressure of the flow of gas in the patient circuit andoutputting a first pressure signal indicative thereof; communicating gasfrom within the patient circuit to ambient atmosphere; and activating aplurality of triggering mechanisms over an expiratory phase of abreathing cycle to increase a sensitivity to a patient initiated triggeras the expiratory phase of the breathing cycle progresses.
 35. A systemfor providing a flow of gas to a patient comprising: a pressuregenerating system adapted to provide a flow of gas to a patientresponsive to a control signal; a patient circuit coupled to thepressure generating system and adapted to communicate the flow of gaswith an airway of a patient; a flow sensor adapted to measure the flowof gas in the patient circuit and to output a first flow signalindicative thereof; a pressure sensor adapted to measure a pressure ofthe flow of gas in the patient circuit and to output a first pressuresignal indicative thereof; an exhaust assembly adapted to communicategas from within the patient circuit to ambient atmosphere; and acontroller that receives the first flow signal and the first pressuresignal and outputs the control signal that controls the flow of gasdelivered to the patient circuit by the pressure generating system and,hence, the flow of gas at a patient's airway, wherein the controllerdetects onset of an expiratory phase of a patient's breathing cycle forcycling an expiratory flow of gas based on such a patient's expiratoryeffort, which is determined based on both the first flow signal and thefirst pressure signal.
 36. The system according to claim 35, wherein thecontroller: determines a patient flow difference (Q_(ref)−Q_(patient)),where Q_(patient) is a current patient flow from the first flow signaland Q_(ref) is a reference patient flow determined from the first flowsignal at a start of a cycling window, which is a period of time duringwhich the expiratory flow of gas from the patient circuit is permitted;determines a patient pressure difference (P_(patient)−P_(ref)), whereP_(patient) is a current patient pressure from the first pressure signaland P_(ref) is a reference patient pressure determined from the firstpressure signal at the start of a cycling window; determines thepatient's expiratory effort as a product of the patient flow differenceand the patient pressure difference; and cycles from providing aninspiratory flow of gas to allowing an expiratory flow of gas from theexhaust assembly responsive to the patient's expiratory effort exceedinga threshold.
 37. The system according to claim 35, wherein thecontroller: determines a patient pressure difference(P_(patient)−P_(ref),), where P_(patient) is a current patient pressurefrom the first pressure signal and P_(ref) is a reference patientpressure determined from the first pressure signal at a start of acycling window; delays a patient flow from the first flow signal todetermine a delayed patient flow; determines a product of the patientpressure difference and the delayed patient flow as the patient'sinspiratory effort; sums the patient's expiratory efforts accumulatedover a time interval; and cycles from providing an inspiratory flow ofgas to allowing an expiratory flow of gas from the exhaust assemblyresponsive to the sum of the patient's expiratory effort exceeding athreshold.
 38. The system according to claim 35, wherein the controller:determines a patient flow difference (Q_(ref)−Q_(patient)), whereQ_(patient) is a current patient flow from the first flow signal andQ_(ref) is a reference patient flow determined from the first flowsignal at a start of a cycling window, which is a period of time duringwhich the expiratory flow of gas from the patient circuit is permitted;determines a patient pressure difference (P_(patient)−P_(ref),), whereP_(patient) is a current patient pressure from the first pressure signaland P_(ref) is a reference patient pressure determined from the firstpressure signal at the start of the cycling window; determines thepatient's expiratory effort as a product of the patient flow differenceand the patient pressure difference; sums the patient's expiratoryefforts accumulated over a time interval; and cycles from providing aninspiratory flow of gas to allowing an expiratory flow of gas from theexhaust assembly responsive to the sum of the patient's expiratoryefforts over the time interval exceeding a threshold.
 39. The systemaccording to claim 38, wherein the time interval is a fixed period oftime.
 40. A method of providing a flow of gas to a patient comprising:generating a flow of gas; providing the flow of gas to a patient via apatient circuit; controlling the flow of gas delivered to a patientresponsive to a control signal; measuring the flow of gas in the patientcircuit and outputting a first flow signal indicative thereof; measuringa pressure of the flow of gas in the patient circuit and outputting afirst pressure signal indicative thereof; communicating gas from withinthe patient circuit to ambient atmosphere; and detecting onset of anexpiratory phase of a patient's breathing cycle for cycling anexpiratory flow of gas based on such a patient's expiratory effort,which is determined based on both the first flow signal and the firstpressure signal.
 41. The method according to claim 40, wherein detectingan onset of an expiratory phase includes: determining a patient flowdifference (Q_(ref)−Q_(ref)), where Q_(patient) is a current patientflow from the first flow signal and Q_(ref) is a reference patient flowdetermined from the first flow signal at a start of a cycling window,which is a period of time during which the expiratory flow of gas fromthe patient circuit is permitted; determining a patient pressuredifference (P_(patient)−P_(ref)), where P_(patient) is a current patientpressure from the first pressure signal and P_(ref) is a referencepatient pressure determined from the first pressure signal at the startof a cycling window; determining the patient's expiratory effort as aproduct of the patient flow difference and the patient pressuredifference; and cycling from providing an inspiratory flow of gas toallowing an expiratory flow of gas from the patient circuit responsiveto the patient's expiratory effort exceeding a threshold.
 42. The methodaccording to claim 40, wherein detecting an onset of an expiratory phaseincludes: determining a patient pressure difference(P_(patient)−P_(ref),), where P_(patient) is a current patient pressurefrom the first pressure signal and P_(ref) is a reference patientpressure determined from the first pressure signal at a start of acycling window, which is a period of time during which the expiratoryflow of gas from the patient circuit is permitted; delaying a patientflow from the first flow signal to determine a delayed patient flow;determining a product of the patient pressure difference and the delayedpatient flow as the patient's expiratory effort; summing the patient'sexpiratory efforts accumulated over a time interval; and cycling fromproviding an inspiratory flow of gas to allowing an expiratory flow ofgas from the patient circuit responsive to the sum of the patient'sexpiratory effort exceeding a threshold.
 43. The method according toclaim 40, wherein detecting an onset of an expiratory phase includes:determining a patient flow difference (Q_(ref)−Q_(patient)), whereQ_(patient) is current patient flow from the first flow signal andQ_(ref) is a reference patient flow determined from the first flowsignal at a start of a cycling window, which is a period of time duringwhich the expiratory flow of gas from the patient circuit is permitted;determining a patient pressure difference (P_(patient)−P_(ref),), whereP_(patient) is a current patient pressure from the first pressure signaland P_(ref) is a reference patient pressure determined from the firstpressure signal at the start of the cycling window; determining thepatient's expiratory effort as a product of the patient flow differenceand the patient pressure difference; summing the patient's expiratoryefforts accumulated over a time interval; and cycling from providing aninspiratory flow of gas to allowing an expiratory flow of gas from thepatient circuit responsive to the sum of the patient's expiratoryefforts over the time interval exceeding a threshold.
 44. The methodaccording to claim 43, wherein the time interval is a fixed duration.45. A system for providing a flow of gas to a patient comprising: apressure generating system adapted to provide a flow of gas to a patientresponsive to a control signal; a patient circuit coupled to thepressure generating system and adapted to communicate the flow of gaswith an airway of a patient; a flow sensor adapted to measure the flowof gas in the patient circuit and to output a first flow signalindicative thereof; a pressure sensor adapted to measure a pressure ofthe flow of gas in the patient circuit and to output a first pressuresignal indicative thereof; an exhaust assembly adapted to communicategas from within the patient circuit to ambient atmosphere; and acontroller that receives the first flow signal and the first pressuresignal, wherein the controller detects onset of an expiratory phase of apatient's breathing cycle, for cycling the system from providing theinspiratory flow of gas to allowing an expiratory flow of gas from theexhaust assembly, responsive to a patient flow determined from the firstflow signal falling below a cycle threshold flow, wherein the controllermonitors patient pressure, via the first pressure signal, at an endportion of the inspiratory phase to determine whether cycling occurredtoo late, and monitors patient flow, via the first flow signal, at abeginning portion of the expiratory phase to determine whether cyclingoccurred too early, and wherein the controller adjusts the cyclethreshold flow for a next breathing cycle responsive to a determinationthat cycling occurred too early or too late.
 46. A method of providing aflow of gas to a patient comprising: generating a flow of gas; providingthe flow of gas to a patient via a patient circuit; controlling the flowof gas delivered to a patient responsive to a control signal; measuringthe flow of gas in the patient circuit and outputting a first flowsignal indicative thereof; measuring a pressure of the flow of gas inthe patient circuit and outputting a first pressure signal indicativethereof; communicating gas from within the patient circuit to ambientatmosphere; and detecting onset of an expiratory phase of a patient'sbreathing cycle for cycling from providing an inspiratory flow of gas toallowing an expiratory flow of gas from the patient circuit, bycomparing a patient flow determined from the first flow signal with acycle threshold flow; cycling from providing the inspiratory flow of gasto allowing the expiratory flow of gas from the patient circuitresponsive to the patient flow falling below the cycle threshold flow;determining whether cycling occurred too late based on the patientpressure at an end portion of the inspiratory phase; determining whethercycling occurred too early based on the patient flow at a beginningportion of the expiratory phase; and adjusting the cycle threshold flowfor a next breathing cycle responsive to a determination that cyclingoccurred too early or too late.